The present invention relates to apparatus for and methods of atomizing liquid coating material for being electrostatically deposited on an article, and in particular to apparatus and methods wherein the coating material is projected from passages formed through a rapidly rotating device for being atomized.
There has been an increasing trend in recent years toward use of liquid paints having a small solvent content and relatively high viscosity for the purpose of preventing environmental pollution. However, to satisfactorily atomize such paints using a rotary atomizing device, it is often necessary to rotate the device at high rotational speeds.
Conventional rotary atomizing devices are usually in the form of a bell or disc, and paint introduced onto a paint feed surface of the device flows in a thin film under centrifugal force to a circular peripheral discharge edge of the device and is atomized as it is projected from the edge. In use of such a device, the degree of atomization of paint is generally in inverse proportion to the thickness of the paint film at the discharge edge, which may be appreciated if it is considered that conventional rotary atomizing devices have smooth paint feed surfaces, so that with increasing thicknesses of the paint film, and considering inertia of the film and air drag on the surface of the film opposite from the paint feed surface, the less likely is the film to be brought up to the speed of rotation of the device by the time it is projected from the edge. On the other hand, film thickness is proportional to the quantity of paint discharged. Consequently, when a conventional rotary atomizing device is used to atomize a relatively large volume flow of paint, atomization suffers and the resulting coating on an article to be painted is often less than satisfactory.
When use is made of a compact rotary atomizing device in which the radius of the device is reduced to reduce its size and weight, it is necessary to significantly increase the rate of rotation of the device to obtain satisfactory atomization of paint or, on the other hand, to significantly reduce the thickness of the paint film supplied to the discharge edge of the device. However, when the rotational frequency of the device is significantly increased, considerable slippage occurs between the paint feed surface of the device and the film of paint, the velocity of the film is considerably less than that of the device as it is projected therefrom, atomization suffers and a large number of bubbles may form on the surface of the coating applied to an article. The bubbles deteriorate the quality of the coating and, if excessive, can spoil the coated article.
One prior effort to overcome the aforementioned difficulties is disclosed in U.S. Pat. No. 4,148,932, issued Apr. 10, 1979. As taught in that patent, atomization may be improved by providing the paint feed surface toward and at the peripheral discharge edge of the rotary device with circumferentially spaced, recessed grooves which extend into the edge in the direction of paint flow. Paint flows over and through the grooves in approaching the discharge edge, and while the patent attributes improved atomization to the formation of a large number of discrete cusps of paint along the whole periphery of the discharge edge, it is believed by the present inventor that improvements in atomization occur primarily because the grooves take a "purchase" on the paint film and bring its velocity toward that of the device by the time it is projected from the edge. However, because the grooves are open channels, paint is still free to slip across and between the grooves as the device rotates, so that the speed of rotation of the paint film at the discharge edge cannot reach that of the device and maximum improvements in atomization are not obtained.